Yeah. Some sort of unusual 80/20 rule is in play. 20% of the attendees keep the other 80% engaged.
Oddly enough if you space things out pseudorandomly like putting name placards of different people next to each other on tables that are far apart, you increase the likelihood of engagement.
If you require people to reach over each other, they're polite and engage.
Never underestimate the power of a few dedicated participants.